Ever since retroreflective coating materials were first commercially provided in the late 1950's, there has been a desire for increased retroreflective brightness from the coatings. The problem has been that the retroreflective elements used in these coating materials, i.e. flour-grain-sized hemispherically metal-coated glass microspheres dispersed in a liquid paint vehicle (see Palmquist et al, U.S. Pat. No. 2,963,378), become randomly oriented when the coating material is applied to a substrate. Typically, only about one-third of the microspheres are aligned with their uncovered portion facing outwardly in an applied coating, and the result is a significant reduction in the intensity with which the coating retroreflects incident light.
The same deficiency exists to a lesser extent in a recently developed system for retroreflectorizing fabrics and other substrates using minute retroreflectorization particles. As taught in a copending application, Bingham and Bailey, U.S. Ser. No. 540,286, filed Jan. 10, 1975, each retroreflectorization particle comprises one or more microspheres arranged as a closely packed monolayer and supported and partially embedded in a binder material that may be softened to adhere the particle to the fabric. The particles are typically cascaded onto the fabric and adhered there in a sparse arrangement which produces a surprising combination of inconspicuousness under ordinary daytime viewing conditions and striking visibility when viewed under illumination by automobile headlamps at night.
However, not all of the retroreflectorization particles cascaded onto a fabric become adhered in position to retroreflect light. The effect is to increase the number of particles that must be applied, preventing even further reductions in daytime conspicuity that would significantly widen the potential scope for such treatments.
The prior art has tried several approaches to increase orientation of retroreflective elements in applied retroreflective coatings. Probably the most successful approach has involved covering the whole surface of microspheres with metal; applying a coating of such microspheres (either by cascading them onto a previously applied, partially dried layer of binder material, or preferably by applying them in admixture in a paint vehicle, which is then dried, see Nellessen et al, U.S. Pat. No. 3,420,597); and then etching away the metal from the top portions of the applied coating of microspheres. However, this method has several disadvantages-- it requires a presently expensive whole-surface coating of microspheres; an extra step after formation of the coating; and, as to sparse coatings on fabrics, a wasteful and possible damaging application of etchant to the large uncovered areas of the fabric. Despite utility for some purposes, this approach has not been widely practiced.
No other approach has provided the necessary characteristics either, and there remains a need for retroreflective coating materials by which retroreflective coatings may be formed in situ on a substrate and yet have a high degree of orientation of retroreflective elements that achieves brilliant retroreflection.